Rear projection screens and light diffusers include light filters which provide an optically dispersing medium for transmitting light from an image source on one side of the screen to a viewer on the opposite side of the screen. A basic refractive light filter has been described in U.S. Pat. No. 2,378,252, which includes a refracting lens system as its principal component. The refracting lens system comprises an array of spherical transparent beads embedded in an opaque binder layer and mounted on a transparent support material. Certain known light filters orient the bead layer toward the image source and the transparent support material toward the viewers. (See, for example, U.S. Pat. No. 5,563,738).
The opaque binder layer affixes the beads to the support material, reduces the reflectivity of the filter, and reduces the amount of light transmitted through the interstices between the beads of the lens system. Light from an image is refracted by the beads and dispersed to the viewer through a transmission area of the beads. This transmission area includes an aperture about the point of contact between the bead and support material and the area surrounding this point where the opaque binder layer is too thin to absorb the refracted light.
Rear projection screens and light diffusers are characterized by their ambient light rejection, resolution, gain, and contrast as properties which are determined by the structure and composition of the component materials. For example, the gain which is a measure of the intensity of transmitted light as a function of the viewing angle, is determined primarily by the index of refraction of the spherical beads and the surrounding medium. Similarly, the ambient light rejection and contrast of the light filter are determined largely by the optical absorption of the binder layer. The resolution of the screen is determined by the size of the beads used and how they pack together in the lens system.
However, the interdependence of certain optical properties and their dependence on the properties of component materials, limit optimization of the optical properties of basic refractive light filters. For example, if the optical absorption of the binder layer is increased to enhance the ambient light rejection of the viewing surface, transmission of refracted image light through the binder layer in the transmission area of the bead will be reduced. In addition, the range of indices of refraction of available materials also limits the performance of such filters. Such interdependencies and material limitations hamper the performance of basic refractive filters.